"K7ITM" wrote in message
oups.com...
Yuri,

It seems to me that when "W8JI" is associated with something, you
assume immediately that it is wrong.

That's what might seem to you, but I point out gross misinformation, when I
come across it. I express my opinion based on what I know or believe. I
could be wrong and I gladly get educated. Mostly, if I see, measure or touch
something, I believe it to be right. Mumbo-jumbo "scientwific explanation",
taking off on tangent to justify the fallacy don't cut it with me.

If you were to read Ronold W. P.
King's explanation about small loop antennas in "Transmission Lines,
Antennas and Waveguides", would you be any more apt to believe it? How
about Glenn S. Smith's discussion of them in Johnson and Jasik's
"Antenna Engineering Handbook" (second edition)? Each of those begins
with a reasonably detailed description of an "unshielded" loop and
moves on to a "shielded" loop.


I don't have the King's book, in Jasik's the treatment of small loops and
shielded loops is dealing with some "medieval" designs. The closest to my
version is his Fig. 5.23a showing balanced shielded loop. But then the
5-23bdoesn't make much sense to me, having small loop on the front of
reflector, when the small loop has the minimum of radiation along the axis
through the loop, and he places the reflector in the minimum - null
direction? The way they show the loops, half of loop solid wire, half coax
line, creates confusion what is antenna, what is shield, or perhaps combines
them. I have not used those designs.

In addition, can you expain to us how the current on the wires on the
inside of the shield is NOT balanced by an equal current in the
opposite direction on the inside surface of the shield? Please tell us
in detail just what currents are where on the shielded loop. If you
are going to try to tell us that some explanation is in error, please
provide us with enough detail that we can make up our own minds. So
far, all I've seen here is some vague reference to confusion about
shields.

The descriptions in each of the two references I gave above are far
more detailed than what you have posted here, either of your own or of
W8JI's, and I find them both enlightening--they are slightly different
from each other--but both detailed enough that you can make up your own
mind about what's really going on, and not have to read ranting
generalities or statements with nothing to back them up.

Cheers,
Tom


I will not get tangled into currents, phasors, but describe my design of
small shielded loop antenna that I used on 160m and this should perhaps shed
some light on the controversy.

I used 1/2" copper water tubing (non ferrous material passing the magnetic
field) for circular loop about 4 foot diameter. At the top the loop had gap,
at the bottom it was mounted in small metallic box. Loop, box and mast were
all DC connected and grounded. Mast was about 5 ft high, with Ham-m rotor at
the base to rotate the contraption. This formed Electrostatic shield for the
antenna.

From the connection box I threaded three turns of electrical house wire #12
and across the ends connected mica trimmer capacitor C1 (abt 1200 pF?) to
resonate the three wire loop antenna at 1.830 kHz). Not connected to
anything else, nor ground or loop.
Then I threaded one turn of the same #12 wire as a coupling turn. One end
was connected to the coax braid, the other end through another mica trimmer
capacitor C2 to the center conductor of the coax. Floating, not grounded or
connected to other loop or tubing.
I tuned the C1 to resonate the three turns at the desired frequency and C2
to provide 50 ohm match to coax. Circuit wise this mirrors the LC parallel
tuned circuit with link coupling and provide better signal than other
published designs.
I tried version of this without copper tubing shield and with. I had local
AC power line noise (within fractions of wavelength) and shielded loop
attenuated the local noise.
The way I see this works, the three turns were the antenna, it was tunable
across the band. The "link" coupling allowed to keep the symmetry of antenna
and provided some isolation for common mode currents between the antenna and
coupling (well known in LC tuned circuit with link coupling.). The copper
tubing was ELECTROSTATIC SHIELD which let's the EM waves pass through.
If the copper tubing IS the antenna, then how does it work? Short, grounded
in the center bent dipole? Then the radiation pattern should have maximum
perpendicular to the plane of the loop/dipole. But the antenna has NULLS in
that direction, corresponding to the properties of the 3 plus 1 wire loops.
You scientwists can play games with theories how it should behave, but the
reality again shows how it behaves. Anyone can build the antenna as I
described and VERIFY it. Wire loops without electrostatic shield tubing
still work the same way as with the shield. So which IS antenna?

Another description of the subject antenna is at
http://www.tpub.com/content/antennaa...-352-14_31.htm

73 Yuri Blanarovich, K3BU, VE3BMV

I haven't gone through this in detail yet, but one misconception is glaring:

Yuri Blanarovich wrote:
. . .
I used 1/2" copper water tubing (non ferrous material passing the magnetic
field) for circular loop about 4 foot diameter. . .

If you believe that, it's no surprise that you're having difficulty
understanding how a shielded loop works.

It's not hard to demonstrate that the (time-varying) magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor. Simply put an oscillator or signal source into a copper box
-- you can solder one op out of PC board material. Run some wires all
around the inside which carry the oscillator signal, putting them as
close to the shield wall as you like. Put a battery inside the box to
power the oscillator and seal the box up. Then sniff around the outside
of the box with any kind of magnetic field detector you can devise. If
you have a little potted oscillator of some kind, you should be able to
do this in a couple of hours at most.

Or, just connect your rig to a good dummy load with some double shielded
coax and sniff around the outside of the copper coax shield. If you put
the detector just outside the shield, the current on the inside of the
shield will be much closer to the detector than the current on the
center conductor. So if the shield is transparent to a magnetic field,
your detector should go wild. (Make sure the rig is very well shielded,
though, so no common mode currents make their way from the rig to the
outside of the shield.)

Alternatively, if you'll spend some time with a good electromagnetics
text learning about eddy currents and the like, you'll understand why
you'd be wasting your time with those experiments.

Once you're convinced that the shield blocks the magnetic as well as
electric field, you'll have to revise your theory on how a shielded loop
works. And you'll find that Tom's explanation is correct.

Roy Lewallen, W7EL

Roy wrote, "It's not hard to demonstrate that the (time-varying)
magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor."

Yes, it's all easy to demonstrate. It's used in practice all the time:
the shielding in a transmitter, the aluminum shield cans around IF and
RF coils, the copper strap around a power transformer (used
specifically to lower the external magnetic field around the
transformer, so it won't couple into low-level audio circuits or affect
colors on a color CRT).

And indeed it all agrees with theory. For this one, you need little
more than Faraday's Law of Magnetic Induction.

It's fine with me if there are people who don't want to be bothered
with theory, but if they profess that something works by means
different from the theory that I understand and which agrees with the
observations I make, they shouldn't expect me to believe them without
putting some very serious effort into explaining why the accepted
theory is wrong. I believe Yuri when he tells me his antenna works.
But I'm not buying into his explanation of HOW it works.

Cheers,
Tom

"K7ITM" wrote in message
ups.com...
Roy wrote, "It's not hard to demonstrate that the (time-varying)
magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that
a time-varying magnetic field will produce a current on a nearby
conductor."

Yes, it's all easy to demonstrate. It's used in practice all the time:
the shielding in a transmitter, the aluminum shield cans around IF and
RF coils, the copper strap around a power transformer (used
specifically to lower the external magnetic field around the
transformer, so it won't couple into low-level audio circuits or affect
colors on a color CRT).

And indeed it all agrees with theory. For this one, you need little
more than Faraday's Law of Magnetic Induction.

It's fine with me if there are people who don't want to be bothered
with theory, but if they profess that something works by means
different from the theory that I understand and which agrees with the
observations I make, they shouldn't expect me to believe them without
putting some very serious effort into explaining why the accepted
theory is wrong. I believe Yuri when he tells me his antenna works.
But I'm not buying into his explanation of HOW it works.

Cheers,
Tom

I am not selling explanations how it works.
I understand your and Roy's points. I am not claiming to try to formulate
the infinitesimal theory of wasaaaap and I didn't try that with loading
coils. Ensuing discussions helped me to better understand the mechanaism of
how things work, the theory and how can I better apply them. I thank you for
that.

What I have problem with someone claiming shield is not a shield (Why do
they bother calling it shield or shielded loop?), when I saw the shielding
properties of it in the vicinity of the local interfering signals. It
performs as a shield to the antenna that is wound inside. Tom categorically
denies SHIELD, it IS the ANTENNA he claims. (Like there is no current drop
along the loading coil! - The gospel from the all-knowing guru.)
What I have problem with someone claiming the small loop antenna (three plus
one turn) is not the antenna, but when I remove the shield, the "not
antenna" is still THE ANTENNA.

I am not arguing the mechanics or theory behind how the shield works, it may
be transparency to magnetic field, it may be the voltage generated in the
gap, bla, bla...
Based on my experience with the said antenna, I concluded that wire loops
are THE antenna, shield works as an electrostatic shield.

I know that if I stick oscillator inside of 10' of 1/2" tubing, I will get
hardly or no signal out. I know if I bend that tubing into a circle with gap
and stick wire loop antenna inside, I can get signals out of that "shielded"
antenna and can attenuate close by interfering signals. Shielding doesn't
MAKE my antenna work (it works without shield too), shield enhances its
rejection/shielding properties in near fields.
I know there are small loops and there are small shielded loops and they
work and I have proved it.

Just don't tell me it is called shield because it is antenna, or that
antenna inside the shield doesn't work, or shield doesn't shield from
electrostatic fields, or that my antenna I described doesn't work as I
described.

Tom may pontificate his ideas to his worshippers, but I don't swallow that.
I point out my, and who else cares, disagreement, especially when I see his
"ideas" migrating into ham literature.

Go ahead with your but, but, butts.....

73
Yuri Blanarovich, K3BU, VE3BMV

Yuri,

Just once, and I'm done with this.

Someone somewhere along the line mistakenly called it a shield. They
didn't understand how it works and what's important. Get over it.
Look at just the "shield" with no wires inside. Isn't that exactly a
single turn loop antenna? Isn't the feedpoint the gap in the loop?

If you put a wire (or several wires) through the inside of that tube
you used to call the shield, they just pick up the signal from the
feedpoint. Consider a single wire through the tube. There is a
voltage across the gap, the feedpoint of the loop. Since there is
essentially no voltage drop along the wire in the center, across the
distance of the gap in the tube, then the voltage across the gap must
appear as transmission line voltage across the coaxial feedlines which
are made up of the wire and the inner surface of the tube. If you've
arranged things symmetrically, then the total gap voltage will divide
equally between the two. Then it's just standard coaxial lines from
there to where you connect your receiver, or where you put a tuned
tank.

Or if you have multiple wires through the tube, the net transmission
line current divides among them. And you can resonate them with a
capacitor, but that doesn't make them have antenna currents on them.

If you have another way to analyze it accurately, fine. I don't care.
My way works for me, and it does not disagree with the _performance_
I've seen you post about. It does disagree with the _theory_ you've
suggested.

As for WHY adding the "shield" helps get rid of local e-field noise
(from sources less than a few wavelengths away, which at VLF might be
kilometers), and why the nulls are more perfect, it's because symmetry
is CRITICAL for that performance, and adding the outside tube allows
you to make a more perfectly symmetrical loop than you can practically
accomplish with just wires and all the tuning stuff you hang off it.
If you are VERY careful to keep things symmetrical, you can also do it
without the tube. But it takes amazingly little imbalance to screw
things up.

Dat's it in a nutshell.

Cheers,
Tom

Yuri Blanarovich wrote:
. . .
Tom may pontificate his ideas to his worshippers, but I don't swallow that.
I point out my, and who else cares, disagreement, especially when I see his
"ideas" migrating into ham literature.
. . .

But Tom's explanation is correct. It's consistent with theory; alternate
explanations aren't. If you're really interested in learning how a
"shielded" loop works and won't accept Tom's explanation because it came
from Tom, you can find a similar explanation in a number of reputable
texts. I'll gladly provide references, if you ask before I leave for
Dayton. Once you gain an understanding of some basic electromagnetic
principles, the correctness of the explanation will be obvious.

Oh, and don't worry about Tom's ideas migrating into the literature.
They were already in the literature well before any of us were born.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
I haven't gone through this in detail yet, but one misconception is
glaring:

Yuri Blanarovich wrote:
. . .
I used 1/2" copper water tubing (non ferrous material passing the
magnetic field) for circular loop about 4 foot diameter. . .

If you believe that, it's no surprise that you're having difficulty
understanding how a shielded loop works.

It's not hard to demonstrate that the (time-varying) magnetic field
doesn't penetrate a non-ferrous shield, if you believe (correctly) that a
time-varying magnetic field will produce a current on a nearby conductor.
Simply put an oscillator or signal source into a copper box -- you can
solder one op out of PC board material. Run some wires all around the
inside which carry the oscillator signal, putting them as close to the
shield wall as you like. Put a battery inside the box to power the
oscillator and seal the box up. Then sniff around the outside of the box
with any kind of magnetic field detector you can devise. If you have a
little potted oscillator of some kind, you should be able to do this in a
couple of hours at most.

That is called Faraday shield and does not function as Electrostatic shield.

Or, just connect your rig to a good dummy load with some double shielded
coax and sniff around the outside of the copper coax shield. If you put
the detector just outside the shield, the current on the inside of the
shield will be much closer to the detector than the current on the center
conductor. So if the shield is transparent to a magnetic field, your
detector should go wild. (Make sure the rig is very well shielded, though,
so no common mode currents make their way from the rig to the outside of
the shield.)

Alternatively, if you'll spend some time with a good electromagnetics text
learning about eddy currents and the like, you'll understand why you'd be
wasting your time with those experiments.

I learned about shieldings, Faradyas, I use them, in equipment design, in RF
and harmonics suppression, I built shielded room for university. But I also
know the difference between the Farady shield and Electrostatic shield and
seen them work. Maybe lumping all shields is as no good as lumping all coils
ain't no good?

Once you're convinced that the shield blocks the magnetic as well as
electric field, you'll have to revise your theory on how a shielded loop
works. And you'll find that Tom's explanation is correct.

Roy Lewallen, W7EL

Roy,
I have magnetothermia machine which is about 200 W push-pull power generator
at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30
inch in circumference. Loop wire, antenna (center conductor of coax) is fed
from the plates of two tubes, shield is open at the far end and grounded at
the exit from the enclosure. I get those 200 W heating my body tissue with
magnetic field. Maybe it has something to do with shielding being a fraction
of a wavelength distance from the radiator and the properties of the
magnetic and electric components in the antenna reactive near field region?

I know that this loop radiates along its circumference, not just from the
gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk?

You seem to associate and stick to wrongos and I am sorry you find their
explanations correct, for the reality proves them wrong.

73 Yuri Blanarovich, K3BU

Yuri Blanarovich wrote:
. . .
I learned about shieldings, Faradyas, I use them, in equipment design, in RF
and harmonics suppression, I built shielded room for university. But I also
know the difference between the Farady shield and Electrostatic shield and
seen them work. Maybe lumping all shields is as no good as lumping all coils
ain't no good?

Sorry, you're not making much sense to me. You said that a non-ferrous
shield is transparent to a (time-varying) magnetic field. The
experiments I proposed illustrate that this is false. This has nothing
to do with what name you attach to a shield.

Roy,
I have magnetothermia machine which is about 200 W push-pull power generator
at around 27 MHz. It uses single turn, shielded loop, made of coax, about 30
inch in circumference. Loop wire, antenna (center conductor of coax) is fed
from the plates of two tubes, shield is open at the far end and grounded at
the exit from the enclosure. I get those 200 W heating my body tissue with
magnetic field.

Hm. How do you know it's from just the magnetic field?

This is really interesting. Just a couple of postings ago, you said that
a non-ferrous shield is transparent to a magnetic field. Now you say
that a magnetic field is heating your body. Do you have some embedded
steel shrapnel or something making your body ferrous, or do you just eat
lots of nails and scrap metal?

Maybe it has something to do with shielding being a fraction
of a wavelength distance from the radiator and the properties of the
magnetic and electric components in the antenna reactive near field region?

What has? The heating? That's due to the lossiness of bodily fluids in
the presence of either time-varying magnetic or electric fields.

I know that this loop radiates along its circumference, not just from the
gap in the shield. What's yer theory? Or it don't (ooops, can't) woyk?

If you'll read what Tom has posted, or a description in any good text,
you'll find that the whole circumference of a "shielded" loop radiates.
The field comes from current on the outside of the "shield", not from
some field penetrating the shield. That's my theory. It's the same as
Tom's, and that of every respected author I've read.

You seem to associate and stick to wrongos and I am sorry you find their
explanations correct, for the reality proves them wrong.

Reality proves Newton wrong -- any fool can see that moving objects come
to rest on their own. There's no conflict between theory and reality --
just between theory and people's interpretations of what they're seeing.

I'll stick with the theory that's been known and confirmed for over a
century. People with alternate theories, like yours, will have to
provide some extraordinary proof to sway my thinking.

It seems you're more interested in proving Tom to be wrong about
something -- anything! -- than taking the effort to really understand
what's actually happening. So nothing else I can post will help you. I
hope the lurkers have gotten something from this, though.

Roy Lewallen, W7EL

Roy Lewallen wrote:

If you'll read what Tom has posted, or a description in any good text,
you'll find that the whole circumference of a "shielded" loop radiates.
The field comes from current on the outside of the "shield", not from
some field penetrating the shield. That's my theory. It's the same as
Tom's, and that of every respected author I've read.

Game, Set, and Match, Roy. The explanation and the everyday application
of the concept of non-ferrous shielding are both simple and elegant.

Seems like the thread stopper to me! I suspect it will continue
anyhow.... 8^)

- 73 de Mike KB3EIA -

Mike Coslo wrote:
The explanation and the everyday
application of the concept of non-ferrous shielding are both simple and
elegant.

I'm fairly ignorant when it comes to shielding. Do the
magnetic fields from a magnet penetrate copper? Do the
magnetic fields from 60 Hz devices penetrate the shield
on coax?
--
73, Cecil http://www.qsl.net/w5dxp